Some communications systems, such as an in-vehicle LAN and the like, operate with a battery as a power source. In this system, in order to reduce the power consumption as small as possible, each node of the communication system transitions from a wake-up mode to a sleep mode when operations of the node are unnecessary. The wake-up mode is a normal operation mode. The sleep mode is a mode in which functions except a restart function are stopped.
One method for transition to the sleep mode will be described, which merely provides background information and does not necessarily constitute the prior art. When a node is prohibited from transitioning to the sleep mode, the node periodically notifies this (transition-prohibited notification) to other nodes via a communication line. When the node is permitted to transition to the sleep mode, the node outputs a transition-permitted notification or stops outputting the transition-prohibited notification. When it is confirmed based on these notifications that all the nodes are permitted to transition to the sleep mode, each node switches its operation mode into the sleep node, so that all the nodes transition to the sleep mode substantially at the same time.
In this method, when at least one node continues outputting the transition-prohibited notification due to a failure, a user's improper use or the like, all the nodes cannot transition to the sleep mode. In this case, the battery may run out accordingly.
For addressing this, a proposed technology monitors a battery voltage to prevent the runt out of the battery. When a battery voltage decrease is detected, this decrease is notified or the node is forced to transition to the sleep mode (see Patent Literature 1)
Patent Literature 1: JP-2005-20570A
However, the above technology is provided to merely prevent the run out of the battery. After the battery is actually run out, the cause of the battery run out cannot be identified.